In recent years, lighting fixtures using solid state light-emitting devices such as light-emitting diodes (LED) and organic electroluminescence (EL) elements are becoming popular as long-life lighting fixtures having low power consumption. For such lighting fixtures, lighting devices are developed which adjust the brightness of the solid state light-emitting device, by controlling a drive circuit of a DC-DC converter by using a pulse width modulation (PWM) signal, the duty ratio of which changes according to a dimming signal.
In the lighting device which performs dimmer control on the solid state light-emitting device according to a PWM signal, the PWM signal is a signal of audio frequency band (for example, 1 KHz). Thus, a choke coil included in the DC-DC converter emits a sound, and the sound is undesirably heard by a user.
Thus, conventionally, a circuit for preventing such sound emission from a lighting device has been proposed (for example, see PTL 1).
PTL 1 discloses a sound emission prevention circuit of a lighting system which performs dimmer control using a PWM signal. The sound emission prevention circuit includes an input terminal for inputting a PWM signal for the dimmer control, and a conversion circuit which converts the inputted PWM signal into a signal at a level dependent on the duty ratio of the PWM signal. According to the configuration, by converting the PWM signal into a DC voltage dependent on the duty ratio of the PWM signal, the PWM signal at audio frequency is not supplied to a drive circuit of the DC-DC converter, thereby preventing the sound emission from a choke coil.